Organic light emitting diodes (OLED) are light emitting diodes in which the emissive electroluminescent material is a film of organic material which emits light in response to an electrical current. The emissive organic layer of an OLED is sandwiched between two electrical contact layers. For enhanced efficiency, in addition to a light emitting layer, the OLED device may incorporate layers of charge transporting material between the emissive layer and the electrical contact layer. These charge transporting layers may comprise either hole or electron transporting materials. These charge transport materials can allow the charge-carrying holes and electrons to migrate through to the emissive layer, thereby facilitating their combination to form a bound state called an exciton. The electrons in the excitons in due course relax into a lower energy state by emitting radiation which, for an OLED device, is of a frequency most often in the visible region.
There is considerable ongoing interest in the development of new materials with improved properties that are suitable for use in the fabrication of OLED devices. Materials that, for example, function as emitters, electron transporters and hole transporters are of particular interest. Many materials have been developed over the years in the attempt to produce improved OLED devices and in particular devices with optimal light output, energy efficiency and life time. In addition, a further notable goal is the realisation of materials that allow the device fabrication process to be simplified. Notwithstanding existing materials, there is a continuing need for materials that have properties such as those identified above that possess superior combination of properties for the fabrication of OLED devices and other electronic devices.
It is known that some reactive mesogens (liquid crystalline materials capable of being chemically crosslinked into a polymer matrix) of the general formula:B—S-A-S—B
where A represents a linear aromatic molecular core comprising a fluorene substituted with two alkyl groups at C-9, S represents flexible spacer units and B represents crosslinking groups such as methacrylate groups, may be useful in the fabrication of organic electronic devices. This is particularly the case if B represents a photo-crosslinkable group, since then the materials function essentially as photoresists, which is to say, thin layers of these materials may be patterned into useful electronic structures by patterned exposure to light, particularly UV light.
Further, if the linear aromatic core A is luminescent in nature, these reactive mesogen materials may be patterned into the active light emitting layers in electroluminescent devices such as organic light emitting diodes (OLEDS) and organic diode lasers. However, working OLED devices of the B—S-A-S—B structure have exhibited disappointingly low lifetimes.
It is an object of the present invention to provide new fluorene containing materials for use in electronic devices which overcome, or substantially reduce, problems associated with existing fluorene derivatives.